THE important fact that certain viruses may be varied in potency, and that protection against one may be afforded by another less potent, is to-day not only gained for science, but has entered the stage of application. It is obvious that great interest attaches, in pursuing this line of study, to the investigation of methods of attenuation adapted to new virus. I announce to-day an advance thus made in regard to rabies.

In passing from a dog to a monkey, and then from one monkey to another, the potency of rabies-virus decreases at each transfer. After its strength has been thus diminished, if the virus is then transferred to a dog, a rabbit, or a Guinea-pig, it still remains attenuated. In other words, it does not regain all at once the intensity of virus from a mad dog. Only a small number of transfers from monkey to monkey is necessary to bring the virus to such a state of attenuation that it will not induce madness in a dog when introduced hypodermically. Even inoculation by trepanning, that most certain method of communicating rabies, will produce no result except that of causing in the animal a condition of insusceptibility to rabies.

The potency of rabies-virus increases in passing from one rabbit to another, or from one Guinea-pig to another. When it has been brought to a maximum in rabbits, it exhibits its full strength on being transferred to the dog, and is then more potent than virus from a mad dog. Such virus inoculated into the circulatory system of a dog invariably causes madness which results in death.

Although the virus rises in potency at each transfer from rabbit to rabbit, or from Guinea-pig to Guinea-pig, it must pass through several of these animals in order to regain its maximum potency, when this has first been reduced in monkeys. In like manner, the virus of a mad dog, which, as I have just stated, lacks much of being of maximum potency, when transferred to the rabbit, must pass through the systems of several individuals before reaching its maximum.

A rational application of the results which I have just made known leads readily to the rendering of dogs insusceptible to rabies. We have learned that the experimenter may have at his disposal attenuated rabies-viruses of different strengths; some which are not fatal will protect the system from the effects of more active viruses, and the latter against those which are fatal. Let us take an example. Rabies-virus is obtained from a rabbit which has died from trepanning after a period of incubation which exceeds by several days the shortest time in which the disease may be induced in the rabbit. This invariably takes place within seven or eight days after inoculation by trepanning with the most potent virus. The virus from the rabbit in which the incubation has been long, is inoculated, by trepanning, into a second rabbit, and the virus from this one into a third. With each successive transfer, some of the virus, which becomes stronger and stronger each time, is inoculated into a dog, who becomes gradually more hardened against the operation of the poison, until he is finally found capable of withstanding a fatal virus. He then becomes entirely insusceptible to rabies, the virus of a mad dog producing no effect upon him, whether introduced by intra-venous inoculation or by trepanning. By inoculation of the blood of rabid animals, under certain conditions, I have succeeded in greatly simplifying the operations of vaccination, and in producing in the dog the most decided state of insusceptibility. I shall soon make known the details of the experiments on this point.

Until the time when rabies shall have become extinct through vaccination, the prevention of the development of this affection, in consequence of bites by rabid dogs, will be a problem of considerable interest. In this direction, the first attempts which I have made give me the greatest hopes of success. The period of incubation after biting is, I have every reason to believe, of such length that the subject may be rendered insusceptible before the fatal form of the disease develops. The preliminary experiments are very favorable to this opinion, but the tests must be infinitely multiplied on various species of animals before therapeutics will have the boldness to try this preventive on man.

Notwithstanding the confidence with which the numerous experiments I have made during the last four years inspire me, I do not announce the facts that point to a possible prevention of hydrophobia without some apprehension. Had I had sufficient material means, I should have preferred not making this communication till I had solicited, by the kindness of some of my associates of the Academy of Sciences and the Academy of Medicine, the verification of the conclusions I have just made known; and I have requested M. Faillières, Minister of Public Instruction, to appoint a commission to which I may submit the dogs I have rendered insusceptible to rabies.

The crucial experiment which I should try at the first opportunity would be to take from my kennels twenty dogs insusceptible to rabies, which should be put in comparison with twenty other dogs. The forty dogs should be caused to be bitten successively by rabid dogs. If the statements which I have made are correct, the twenty dogs deemed by me insusceptible will all escape, while the other twenty will be attacked by rabies. In a second experiment, not less decisive, forty dogs would be used, of which twenty had been previously vaccinated, and the others had not. The forty dogs should be trepanned with the virus of a mad dog. The twenty vaccinated dogs would escape, and the other twenty would all die of rabies, with paralysis, or with mania.—Translated for the Popular Science Monthly from the Revue Scientifique.

↑Communicated to the Academy of Sciences, May 19, 1884, by M. Pasteur and MM. Chamberland and Roux.